1. Field of the Invention
The present invention relates to a lighting device in which blue or ultraviolet light is emitted from a light-emitting element, such as a light-emitting diode (LED), and excites fluorescent substances of different colors to produce light of different colors, so that by mixing the different colors, illumination light is emitted.
2. Description of the Related Art
Generally available LEDs have a structure in which an LED chip is encapsulated in transparent molded resin. An LED of this type is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 5-152609. This LED is formed by mixing a yellow fluorescent substance (which is mainly excited by blue light to thereby mainly emit yellow light) into molded resin in which an LED chip capable of emitting blue light is sealed.
The disclosed LED emits white light of high color rendering properties by mixing the blue light emitted by the LED chip with the yellow light emitted by the yellow fluorescent substance excited by the blue light.
However, in the disclosed LED, since the molded resin has the double function of sealing the LED chip and acting as a fluorescent layer, if the material and thickness of the molded resin and the concentration of the yellow fluorescent substance are designed to optimize one of the functions, the other function may be less than optimal. For instance, if the LED chip is encapsulated in molded resin of an optimal thickness for preventing ingress of moisture or corrosive gases, the efficiency of color conversion (wavelength conversion) of the fluorescent layer may not be optimal.
Further, in the disclosed LED, since the yellow fluorescent substance is mixed into the molded resin, if defects occur in the yellow fluorescent substance or air bubbles occur in the molded resin, the LED must be rejected. In other words, the fact that the molded resin of the disclosed LED has a double function reduces product yield.